Sara, labelled Cerdita[a] by bully Maca and her friends Roci and Claudia, is an overweight teenage girl living in a small town in Extremadura. One day during the summer, Sara decides to visit the local swimming pool, only to be discovered by the three bullies, who torment her and steal her backpack and clothes. While walking home, she is harassed by a group of men and escapes onto a side road, where she sees a parked van which, initially unbeknownst to her, an unnamed man is using to kidnap her bullies. Claudia appears through the rear window, but Sara ignores her pleas for help as the kidnapper, who was also present at the pool, leaves her Claudia's towel before driving off. Sara returns home, deciding not to tell anyone about what happened.News reaches the town that the pool's lifeguard has been found dead, and that its waitress has gone missing, along with the three girls. Sara and her mother Asun go to the pool, where Sara is questioned by both Asun and the local Civil Guard about what she saw, and denies having been at the pool that afternoon. Later that night, Sara realises that she can track her mobile phone, which the girls had stolen the previous day, to help determine the whereabouts of the bullies. She steals her father's phone and ventures into the forest, where she manages to locate her backpack and phone. In doing so, she has a second encounter with the kidnapper, who warns her to be quiet and begins to display some affection towards her before escaping. At the same time, some family members of the missing girls, including Claudia's mother Elena, undertake their own investigation in the forest, during which they find the body of the missing waitress.After Sara returns home, Pedro, a friend of the missing girls, comes to her house and invites her out. He introduces her to marijuana and reveals that he knows she was lying about not being at the pool on the day the girls disappeared. He also reveals that the townspeople suspect him to be responsible for the disappearance, and Sara promises to disclose what she saw to the Civil Guard if he is arrested. They return to the town, where a hysterical Elena attacks Sara before Pedro reveals her truth to the assembled crowd. Sara is taken in by the Civil Guard for questioning and begins to tell them the information she has, but the sudden onset of her period prevents further interrogation.Unbeknownst to Sara and Asun, the kidnapper has come to their home and attacked Sara's father. Sara and Asun have a heated argument, during which the kidnapper incapacitates Asun and takes Sara in his van to a warehouse in an unknown location. There, Sara discovers Roci and Claudia, who are still alive but have been gagged and hung from hooks by their hands. She removes the gags and attempts to untie them, but the kidnapper returns before she can do so. She discovers Maca's decomposing body while attempting to flee, and is found by the kidnapper, who embraces her and tries to get her to kill the girls. She instead attacks him, and in the chaos he accidentally fires his shotgun, blowing off Claudia's hand. Sara kills the kidnapper by biting a chunk out of his neck. Sara then picks up the shotgun and contemplates killing her former bullies, but instead shoots the chains suspending them. She leaves and begins walking, soon encountering Pedro on a motorbike, ultimately riding with him back to the town. (thanks to Wikipedia)